1. Field of the Invention
The present invention relates to a scanning optical apparatus and an image-forming apparatus using it and, more particularly, is suitably applicable to apparatus, for example, such as laser beam printers, digital copiers, etc. involving the electrophotographic process, constructed to reflectively deflect light optically modulated and emitted from a light source means, by a deflecting element consisting of a rotary polygon mirror or the like (i.e., deflect the light to scan) and thereafter optically scan an area on a surface to be scanned, through an imaging element with the f-characteristic to record image information on the surface.
2. Related Background Art
In the scanning optical apparatus such as the laser beam printers (LBPs) and the like heretofore, the image recording was carried out in such a way that the light optically modulated and emitted according to an image signal from the light source means is periodically deflected by an optical deflector, e.g., consisting of a rotary polygon mirror (polygon mirror) and is condensed in a spot shape on a surface of a photosensitive recording medium (photosensitive drum) by an imaging optical system with the fxcex8 characteristic to optically scan the area on the surface.
FIG. 13 is a schematic diagram to show the major part of a conventional scanning optical apparatus.
In FIG. 13 a diverging beam emitted from light source means 91 is converted into a nearly parallel beam by a collimator lens 92 and the beam is limited by a stop 93 to enter a cylindrical lens 94 having a predetermined refracting power only in the sub-scanning direction. The nearly parallel beam incident to the cylindrical lens 94 emerges in the original state in the main scanning section. However, the beam is condensed in the sub-scanning section to be focused as an approximately linear image on a deflection facet (reflective surface) 95a of an optical deflector 95 consisting of a polygon mirror.
The beam reflectively deflected by the deflection facet 95a of the optical deflector 95 is then guided through an imaging optical system with the fxcex8 characteristic (fxcex8 lens system) 96 onto a photosensitive drum surface 98 as a surface to be scanned. The optical deflector 95 is rotated in the direction of arrow A to optically scan the area on the photosensitive drum surface 98 in the direction of arrow B, thereby recording the image information thereon.
In order to record the image information with high accuracy, the scanning optical apparatus of this type needs to meet requirements that the curvature of field be corrected for well throughout the entire surface to be scanned, so as to maintain the spot size uniform and that the optical system have such distortion (fxcex8 characteristic) as to establish a proportional relation between angle of incident light and image height.
In addition to this, it is recently becoming important in order to implement high-definition printing, (1) to equalize the spot diameter in the sub-scanning direction in an effective image area (effective scanning area); (2) to equalize the spacing between adjacent pitches in the effective image area in the case of multi-beam scanning; and so on, and it is necessary to equalize the F-number (Fno) in the sub-scanning direction of the beam incident to the surface to be scanned, in the effective image area.
Japanese Patent Application Laid-Open No. 8-297256, which was filed previously by the assignee of the present application, describes the technology of equalizing Fno in the sub-scanning direction of the beam incident to the surface to be scanned, in the effective image area, by changing radii of curvatures in the sub-scanning direction of at least two lens surfaces of lenses constituting the image optical system (third optical element), from on the axis toward off the axis in the main scanning direction.
An object of the present invention is to provide a compact scanning optical apparatus optimal for high-definition image recording and an image-forming apparatus using it, which can equalize the spot size in the sub-scanning direction in the effective scanning area and equalize the adjacent pitch spacing in the effective scanning area in the case of multi-beam scanning, by making up a third optical element of two or more lenses, continuously changing radii of curvatures in the sub-scanning direction of a lens surface of at least one lens out of the two or more lenses, from on the axis toward off the axis in the main scanning direction, setting a rate of change of the curvatures in the sub-scanning direction within one beam on the lens surface to not more than 10%, and determining the shape of the lens having the lens surface in the main scanning direction so that the F-number in the sub-scanning direction of the beam incident to the surface to be scanned, becomes approximately constant in the effective scanning area.
A scanning optical apparatus according to one aspect of the present invention is a scanning optical apparatus comprising a deflecting element for reflectively deflecting a beam emitted from light source means, into a main scanning direction, and an optical element for focusing the beam reflectively deflected by the deflecting element, on a surface to be scanned,
wherein said optical element comprises two or more lenses, radii of curvatures in a sub-scanning direction of at least one lens surface of at least one lens La out of said two or more lenses continuously change from on the axis toward off the axis in the main scanning direction, a rate of change of the curvatures in the sub-scanning direction within one beam on the lens surface is set to not more than 10%, and a shape of the lens La in the main scanning direction is determined so that F-numbers in the sub-scanning direction of the beam incident to the surface to be scanned are approximately constant in an effective scanning area.
In the scanning optical apparatus according to another aspect of the invention, the shape of the lens La in the main scanning direction is determined so that at least one lens surface has a center of curvature on the deflecting element side.
In the scanning optical apparatus according to another aspect of the invention, said lens La is located at a position closest to the surface to be scanned, out of the two or more lenses constituting said optical element.
In the scanning optical apparatus according to another aspect of the invention, the following condition is satisfied:
xcfx86mL less than xcfx86sL,
where xcfx86mL is a power of said lens La in the main scanning direction and xcfx86sL a power thereof in the sub-scanning direction.
In the scanning optical apparatus according to another aspect of the invention, the following condition is satisfied:
xcfx86mL/xcfx86mf less than 0.5,
where xcfx86mL is a power of said lens La in the main scanning direction and xcfx86sL a power of the whole of said optical element in the main scanning direction.
In the scanning optical apparatus according to another aspect of the invention, the following condition is satisfied:
0.5 less than xcfx86sL/xcfx86sf,
where xcfx86sL is a power of said lens La in the sub-scanning direction and xcfx86sf a power of the whole of said optical element in the sub-scanning direction.
In the scanning optical apparatus according to another aspect of the invention, said lens La is disposed on the surface-to-be-scanned side with respect to a middle point between a deflection point on said deflecting element and said surface to be scanned.
In the scanning optical apparatus according to another aspect of the invention, said lens La is made by plastic molding.
In the scanning optical apparatus according to another aspect of the invention, image magnifications in the sub-scanning direction are approximately constant in the effective scanning area.
In the scanning optical apparatus according to another aspect of the invention, said light source means comprises a plurality of light emitting regions.
In the scanning optical apparatus according to another aspect of the invention, the beam emitted from said light source means is focused in the form of a linear beam longer than one deflection facet out of a plurality of deflection facets of the deflecting element in the main scanning direction, on the deflection facet.
An image-forming apparatus according to still another aspect of the present invention is an image-forming apparatus comprising either of the scanning optical apparatus as set forth, and a printer controller for converting code data supplied from an external device, into an image signal and then entering the image signal into said scanning optical apparatus.
In the image-forming apparatus according to another aspect of the invention, an image is formed by background exposure.